1. Field of the Invention
The present invention relates generally to a miniature X-ray device. More specifically, the present invention relates to an X-ray catheter. More specifically, the present invention relates to an X-ray device having a braze joint between electrodes and insulating materials.
2. Background Art
Cardiovascular diseases affect millions of people, often causing heart attacks and death. One common aspect of many cardiovascular diseases is stenosis, or the thickening of an artery or vein wall, decreasing blood flow through the vessel. Angioplasty procedures have been developed to reopen clogged arteries without resorting to a bypass operation. However, in a large percentage of cases, arteries become occluded again after an angioplasty procedure. This recurrent thickening of the vessel wall is known as restenosis. Restenosis frequently requires a second angioplasty and eventual bypass surgery. Bypass surgery is very stressful on the patient, requiring the chest to be opened, and presents risks from infection, anesthesia, and heart failure.
One method of treating restenosis includes using miniature X-ray devices to irradiate blood vessels and other human body cavities. An X-ray catheter is comprised of a coaxial cable and a miniature X-ray emitter-connected to the cable's distal end. The proximal end of the coaxial cable is connected to a high voltage power source. The X-ray emitter consists of an anode and a cathode assembly mounted in a miniature shell (tube), made of an insulator with very high dielectric strength. Typically, the anode is comprised of platinum, tungsten, or another heavy metal.
To activate the emitter, high voltage is applied between electrodes. A high electric field is generated at the cathode surface and causes field emission of electrons. Emitted electrons are accelerated by the electric field and impinge on the anode. As the electrons strike the anode, X-ray energy is produced and radiated. The radiation occurs as high-speed electrons are slowed or stopped by passing near the positively charged nuclei of the anode material, or, as incoming electrons collide with the anode atoms and knock the electrons near the anode nuclei out of orbit and replacing the knocked out electrons with other electrons.
For adequate production of X-ray, a high voltage source supplies the catheter with voltage in the range of 15 to 30 kV and current in the range of 10 to 100 μA. For coronary applications, the outer diameter of an X-ray emitter must be as small as 1.00 to 1.25 mm. Thus, specific material properties and characteristics are desired for each element in the emitter.
A material used for the shell of an X-ray emitting catheter must possess a very high dielectric strength (120-200 kV/mm) combined with high electrical resistivity (1015 Ohm-cm), gas impermeability, and moderate mechanical strength. A brazing process bonds the shell to the anode. In order to reliably join with the anode, the shell material should have a coefficient of thermal expansion (CTE) close to that of the anode and the braze layer that joins the anode and the shell. Finally, the shell material should have low to moderate absorption of X-ray within the energy range of 10-20 kV. Thus, the material should be composed of relatively low weight elements.
The shell must be hermetically sealed to the anode and cathode. Typically, this is done using a brazing process. The shell-to-cathode joint is relatively easy to obtain, and can employ a butt joint. The cathode is electrically connected to the metallic coating outside the shell, and there is no voltage applied to or through the joint.
However, the shell-to-anode joint is more difficult. The anode must be placed inside the shell and the joint should be able to withstand high voltage applied between the outer surface of the shell and the anode. The joint is typically exposed to an electric field of about 100 to 150 kV/mm. This strong electric field imposes several very strict requirements not only on the material itself, but also on the quality of the joint. For instance, any voids formed in the brazing layer enhance the electric field, which leads to dielectric breakdown. Additionally, any sharp points of braze or any spill of excess braze also enhances the electric field, again causing dielectric breakdown.
In order to avoid voids, points or spills, it is critical that the braze material not flow beyond the braze area. Brazing of emitters for catheter X-rays creates real limitations on the volume of materials that can be used in a braze joint. Thus, the quantity of material used in a braze joint must be carefully controlled. Conventionally, braze preforms are made and placed on the braze surfaces prior to the brazing process. However, because it is difficult to fabricate braze preforms having a thickness of 25 microns or less, the amount of braze material applied to the brazing joint typically exceeds the amount of braze material needed. Thus, overflow and spills are not uncommon when brazing catheter emitters. This leads to an increased electrical field during use and ultimately results in dielectric breakdown.
Furthermore, mass production of X-ray emitters using very small braze preforms is difficult to achieve because each braze preform must be individually placed on the brazing surfaces of the catheter emitters. This is a time consuming and difficult process.
Two types of brazing currently exist: metal brazing and nonmetal brazing. Metal brazing consists of placing a first easy-to-melt metal between two metals with higher melting points. The metals are heated until the low-melting point metal liquefies. While melted, the first metal bonds with the layers of the second metal, creating the braze. Upon cooling of the heated pieces, the pieces are inseparable, as they have been fused together.
The second type of brazing is for nonmetals. This type of brazing consists of brazing nonmetal materials, such as quartz or alumina, to a metal by adding an active metal, such as titanium or zirconium, to the braze material. The active metal is attracted to and reacts with the nonmetal, creating a chemical bond. This brazing process is referred to as active brazing. Active brazing usually requires high temperatures to liquify or dissolve the active metal to enable reaction between the active metal and the nonmetal. Quartz and alumina are examples of nonmetals that are brazed using active metals.
Thus, what is needed is a low temperature method of brazing that creates a strong chemical bond with a nonmetal surface. Further, what is needed is an easy way to manufacture batches of catheter X-ray emitters while carefully controlling the volume of braze material applied for a braze joint.